Anti-terrorism chiefs were so fearful that suicide bombers would attack Buckingham Palace and the Houses of Parliament that they banned anyone from entering or leaving the buildings for 90 minutes, it was revealed yesterday.

The centres of British power, as well as New Scotland Yard, were believed to be potential targets in the aftermath of the July 7 2005 bombings in London.

The revelation came from Peter Clarke, the former head of the anti-terrorist branch at Scotland Yard, as he gave evidence at the inquest into the death of Jean Charles de Menezes at Stockwell tube station on July 22 that year.

All three buildings, which are classed as "iconic" targets for terrorists, were locked down for an hour and a half, Clarke revealed. He was trying to impress on the jury the tense atmosphere that hung over the capital in the run-up to the shooting dead of De Menezes by police who mistook him for a suicide bomber.

Clarke said the lockdown happened on July 12 2005, after a bomb factory used to make the July 7 bombs was discovered in Leeds, followed by police finding a car at Luton train station whose boot contained explosive materials and a firearm. CCTV from the station suggested a possible fifth suicide bomber was on the loose.

The former anti-terror chief said: "It was completely unprecedented, as was some of the decision-making having to be made at that time about whether to warn the public about the possibility of a suicide bomber being on the loose or not.

"I remember those as being some of the most difficult decisions that one had ever confronted. If we warned the public, we could cause unnecessary panic. If we didn't and something terrible happened, the obvious question is: why didn't you warn the public?

"That is the sort of pressure we were working under day in, day out. July 12 is but one example."

Clarke, who headed Scotland Yard's anti-terrorism command at the time of the July 7 attacks, revealed that his 16-year-old son was nearly caught up at the scene of one bombing, at King's Cross. Clarke advised him to get a bus, minutes before one was blown up by a suicide bomber.

His son was heading to Cambridge and called his father, who was at home, to tell him of the chaos at King's Cross. Unknown to police at that time, a bomb had been exploded on an underground train between King's Cross and Russell Square stations.

Clarke told the jury: "He phoned me at home and said, 'What should I do, there appears to be smoke and people running.'

"So I gave him the instruction to get away from there as quickly as possible and in fact we, my wife and I, told him to get on a bus to get away."

Clarke told the inquest that his wife had probably suffered delayed shock after her son's narrow escape. Clarke pulled out of a family holiday, but had then hoped to finally get some leave to comfort his wife.

He finally set off to join his family on July 21, only to be called back to duty after the attempted second set of suicide bomb attacks.

Clarke also told the inquest that after the July 7 attacks officers had been issued with instructions on how a suicide bomber might be recognised. They were told to look out for people who were sweating. mumbling, praying, were recently-clean shaven, looking anxious or "wearing bulky clothing not in keeping with the weather or event", and also to "look for a wire or toggle protruding from an overtly carried bag".

In the event of coming across a suspect, officers were told to call for armed officers and bomb disposal experts.